In the petroleum refining industry, the fluidized catalytic cracking (FCC) of heavy oil feeds is well established and a number of different cracking processes and units are available to the industry. The process may be oriented to conversion of the feed to gasoline or to other desired products such as the light olefins used as petrochemical feedstocks. The parameters of the FCC process are well-known and the technical literature is replete with discussion of the process.
Normally, suitably preheated, relatively high molecular weight hydrocarbon feeds such as vacuum gas oil or gas oil and resid are contacted with hot, finely-divided, solid catalyst particles in a fluidized bed reaction zone which now is conventionally in the form of an elongated upright riser. The catalyst/oil mixture is maintained at an elevated temperature in a fluidized state for a period of time sufficient to effect the desired degree of cracking to the desired lower molecular weight hydrocarbons such as those typical of those present in motor gasolines and distillate fuels.
During the cracking reaction, coke is deposited on the catalyst particles, reducing the activity of and selectivity of the catalyst. In order to restore activity, the catalyst is transferred from the reaction zone into a regeneration zone where the spent catalyst is maintained as a fluidized bed by the upward passage of an oxygen-containing regeneration gas, such as air, under conditions to burn at least a portion, preferably a major portion, of the coke from the catalyst. The regenerated catalyst is subsequently withdrawn from the regeneration zone and reintroduced into the reaction zone for reaction with additional hydrocarbon feed. The regeneration serves not only to remove the accumulated coke from the catalyst to restore its activity but also to supply heat from the exothermic combustion process in the regenerator for the endothermic cracking in the reactor so as to maintain an overall operating heat balance for the unit.
In a fluid catalytic cracking unit (FCCU), the prevailing commercial practice is to employ teed injector nozzles which atomize the heavy oil feed and inject into the feed zone at the base of the riser. It is typical to dispose a number of injectors around the periphery of the riser, the exact number depending on the size of the riser and the unit, for example, four, six or eight. The injectors typically atomize the feed with the aid of steam. The injectors are typically disposed at one level in the riser. Much effort has been expended in the design of the injector nozzles with the objective of improving catalyst/oil mixing and a number of different proprietary nozzle designs exist, each with its own asserted advantages. This focus of effort, however, has only addressed one side of the problem, namely the injection of the oil; in order to address the mixing problem more completely, it is necessary to consider also the effect of the catalyst on the mechanism of the mixing.
In the FCC unit, the regenerated hot catalyst from the regenerator is transported downward from the regenerator in a standpipe and then through a bend (e.g., J-bend or U-bend) and lifted upward into the feed zone of the FCC riser by a current of steam at the base of the riser. The feed is generally radially injected into the feed zone of the FCC riser reactor, with nozzles arranged around the circumference of the riser. It is generally assumed that the solids distribution in the standpipe and feed zone is uniform and the feed nozzle design is also based on the assumption of uniform solids distribution in the feed zone. Observation has shown, however, that the feed zone may be far from uniform. Poor oil-catalyst contacting arising from the non-uniformity catalyst distribution and non-even distribution of the feed rate to each feed nozzle with radial feed injection may lead to excessively hot and cold regions in the riser feed. This results in poor yield selectivity, higher dry gas and butadiene make, and increased MCB (main column bottom) yield at constant coke yield. Non selective thermal cracking in the hot regions of high cat/oil ratio raises the amount of low value light gases. Incomplete feed vaporization in the cold regions with low cat/oil ratio increases MCB yield at constant coke yield.
Because the FCCU is probably the single most important generator of valuable fuels and chemical products in a refinery, even minor variations of yields can have a significant impact on economics. Constant optimization of the unit is therefore necessary and the FCCU is normally operated under multivariable constraint control to maximize refinery profits on a continuous basis with product yield and quality being important factors in overall profitability. Optimization of the feed zone has, up to the present been carried out on a trial-and-error basis using the experience of the unit operators and engineers in an attempt to secure the operating conditions needed for the best unit economics. This existing approach does not result in optimal feed/catalyst distribution in the feed/mixing zone and in view of the large throughput of these units, even relatively short-lived departures from the optimal conditions may result in significant losses in profitability. The problem facing the prudent refinery operator, therefore, is to systematize the operation of the feed/mixing zone to secure optimally uniform mixing.